Pure and mixed cultures of the coaggregating pair; Fusobacterium nucleatum and Streptococcus sanguis (CC5A) have been grown successfully in both the batch and continuous modes. Pure culture studies revealed that F. nucleatum used glutamic acid and glucose concurently during exponential growth in batch culture while less than half the glucose, and all of the glutamic acid were used in continuous culture. Glutamic acid seemed to be used in preference to glucose by F. nucleatum. Lysine was used after glucose and glutamic acid during batch growth of F. nucleatum. A stable mixed culture, including extensive coaggregation of these organisms was obtained when CC5A was inoculated into the continuous culture medium before F. nucleatum. In contrast, using the same conditions, the growth of CC5A was suppressed if F. nucleatum was inoculated first. When growth together in continuous culture, F. nucleatum was reduced to very low levels while CC5A became dominant as the dilution rate was raised. F. nucleatum remained in the culture and its increase was reflected by an increased butyric acid production after the dilution rate of the culture was reduced. Butyric acid production proved directly related to the growth rate of F. nucleatum. In pure culture, two rates of cell growth and butyric acid formation were seen, while in a batch grown mixed culture, only a single rate of butyric acid production was observed. The interactions of the two organisms during butyrate production are unknown and under investigation.